1. Field of the Invention
This invention relates to capacitance gauging for the direct measurement of surface texture.
2. Description of the Prior Art
U.S. Pat. No. 4,103,226, "APPARATUS FOR GAUGING THE TEXTURE OF A CONDUCTING SURFACE"--Fromsen, et al., and U.S. Pat. No. 4,130,796, "CALIBRATING AND MEASURING CIRCUIT FOR A CAPACITIVE PROBE-TYPE INSTRUMENT"--Shum provides detailed descriptions of an electronic instrumentation package that may be used with the probe of the instant invention. The disclosures of these patents are hereby incorporated by reference.
The invention is in the field of surface texture measurement, as more fully described in the American National Standard publication, SURFACE TEXTURE, ANSIB46.1-1978, published by the American Society of Mechanical Engineers, the contents of which are herein incorporated by reference.
Surface texture includes waviness, roughness (superposed on waviness), lay, and flaws; it is the repetitive of randon deviations from an intended (nominal) surface finish and is generally associated with surfaces produced by such means as abrading, casting, coating, cutting, etching, plastic deformation, sintering, wear, erosion, and the like. For purposes of this disclosure, a glossary of terms (similar to those provided in the above publication) are listed as follows:
Waviness is the more widely spaced component of surface texture. Unless otherwise noted, waviness is to include all irregularities whose spacing is greater than that designated as surface "roughness". Waviness may result from such factors as machine or work deflections, vibration, chatter, heat treatment or warping strains.
Roughness consists of the finer irregularities of the surface texture, usually including those irregularities which result from the inherent action of the production process. These are considered to include transverse feed marks (such as caused by lathes) and other irregularities of the work surface. Roughness may be considered superposed on a "wavy" surface.
Lay is the direction of the predominant surface pattern, ordinarily determined by the production method used.
Flaws are unitentional irregularities which occur at one place or at relatively infrequent or widely varying intervals on the surface. Flaws include such defects as cracks, blow holes, inclusions, checks, ridges, scratches, etc.
A peak is the point of maximum height on that portion of a profile which lies above the centerline and between two intersections of the profile and the centerline.
A valley is the point of maximum depth on that portion of a profile which lies below the centerline and between two intersections of the profile and the centerline.
Roughness Average Value (R.sub.a) may be obtained when using stylus-type measurement instruments. It is the arithmetic average of the absolute values of the measured profile height deviations within a sampling length and measured from the graphical centerline, as the stylus moves relative to the surface being tested. With the probe of the instant invention, capacitive measurement is made of volume deviations of a sampled surface area, such that three-dimensional surface "voids" correspond to the previously defined two-dimensional "valleys". Hence, for purposes of this disclosure, an average three-dimensional surface assessment is obtained which corresponds to the two-dimensional R.sub.a.
For a fuller understanding of these terms, one may refer to the drawings.
A gauging system for surface texture which is based upon capacitance should include a probe that allows the active sensor area, acting as one plate of a capacitor, to conform to the surface (acting as the other plate of the capacitor) being assessed. For accuracy, this active sensor area should be spaced from the conductive surface by a fixed thickness of dielectric insulation. The probe should provide sensor rigidity such that the active sensor area does not settle into the "valleys" of the surface, while also providing a means of connection between the active sensor area and the probe output point which does not change in capacitance and/or resistance.
3. Problems with Prior Art Apparatus
The probe of U.S. Pat. No. 4,103,226 is exemplary of the prior art in which problems with accurate measurement repeatability have been detected. In one probe, wire leads are connected to the backs of sensor plates and are run through or around a nonconductive foamed elastomer backing for the plates and then threaded through an opening in a casing for connection to an electronic instrumentation package. The leads of this probe are fragile and present some difficulty when replacement of the sensor portion (a "wear" item) is necessary. The fragile connections between the leads and sensor plates are subject to damage during sensor replacement. In another probe, the sensor plates are secured (as by a conductive adhesive) to a conductive elastomer backing with contacts. An insulating material (non-conductive elastomer) spaces the contacts and conductive elastomer from each other and from the casing; and leads are run from the contacts, through the insulating material and casing, for connection to the electronic instrumentation package. Again, connection fragility and replacement difficulty have been encountered. Proper use of the probe depends on a uniform distribution of the forces normal to the sensor plates when the probe is pressed to the workpiece. The two different materials (insulating material and conductive elastomer) present problems of relative movement, such as shifting, to redistribute these normal forces and produce a pressure gradient, which greatly affects the measurement results.
These two prior art probes have an additional problem in common. The insulated spacing of the contact plates and the workpiece is provided by a dielectric which is attached to the face of each plate by an adhesive. In practice, adhesive tape, such as common "scotch tape", is applied to the face of each plate to act as the adhesively attached dielectric. For stable, predictable, repeatable measurements, the distance between the sensor plates and the workpiece must be constant over the whole area of the active sensor. Adhesives can and do flow and redistribute. For instance, when "scotch tape" has been used as the adhesively attached dielectric, heat and/or stress has caused the adhesive to flow and redistribute so that the distance between the contact plates and the workpiece has varied over the area of the plates and has affected greatly the accuracy of the surface texture measurement. Also in practice, it has been found that the edges of the sensor plates can cut into the dielectric, causing the nuisances and inaccuracies of adhesive sticking to the workpiece.
Other problems encountered in the prior art probes involve system interconnection. These probes are connected to an instrumentation package, usually by a coaxial cable. Because the cables have an inherent capacitance which is directly proportional to the cable length, it has been preferable to reduce this capacitance by reducing the length of the cable. Additionally, small changes in the capacitance of the cable can adversely affect the system when gauging or measuring surface texture in which even smaller changes in the capacitance need to be measured.
Usually, prior to measuring the texture of a particular work surface, the system is calbrated on a reference surface which may not have the exact radius of curvature of the workpiece. According to the size and shape and conformability of the active sensor area, greater or lesser active sensor area may be presented to the reference surface than to the work surface, so that an incorrect measurment results.
Although prior art probes have been provided with flexible active sensor areas, they have not been sufficiently flexible to adequately and easily conform to three-dimensional curvatures, as in work surfaces having compound radii of curvature. With prior art devices for gauging a surface characteristic by capacitive coupling, a variation of capacitance is sensed as the probe approaches the work surface prior to proper seating of the probe on the work, and no means for sensing this proper seating prior to commencing the gauging has been provided.
An object of the instant invention is to provide a surface texture measurement probe in which the distance between the active sensor area and the workpiece will remain constant with repeated use.
Another object of the instant invention is to provide a secure electrical connection between the active sensor area and the electronic instrumentation to reduce or eliminate changes in capacitance and resistance.
A further object of the instant invention is to provide a probe without the above-mentioned problems of normal force pressure redistribution.
Still further, it is an object of this invention to provide an easily replaceably, less fragile sensor portion for a surface texture measurement probe whereby duplication of resistance and capacitance characteristics are obtainable with a high degree of certainty.
Additionally, it is an object of the instant invention to provide a probe in which a flexible active sensor is thin enough (on the order of several angstroms) to eliminate the "cutting edge" effect, while providing a secure, predictable connection between it and the electronic instrumentation.
It is a further object of the invention to reduce or eliminate from the system the adverse effects of changes in the inherent capacitance of coaxial cable, while enabling use of cables of greater length.
An additional object of the invention is to provide a means for electrically masking a portion or portions of the sensor to define a particular active sensor area configuration.
Another object of the invention is to provide a method and means for sensing proper seating of a gauging probe with a conductive work surface which is flat or has a compound curvature, and to commence gauging a predetermined characteristic of the surface only after proper seating has been accomplished and sensed.
Further, an object of the instant invention is to provide an active sensor area which more easily and adequately conforms to the work surface.
These and other objects will become readily apparent from the following disclosure.